In recent years, as charging methods of charging a photosensitive member in an image forming apparatus that employs an electrophotographic process, for example, methods of charging a photosensitive member by causing a charging member as charging means, such as a charging roller, to charge it while being in contact with or adjacent to a surface of the photosensitive member have been known.
One known example of such charging methods is a DC charging method of charging a charging member by applying a charging voltage solely consisting of a direct-current voltage to it. Another known example is an AC charging method of charging a charging member by applying a charging voltage in which a direct-current voltage and an alternating-current voltage are superimposed to it, converging a potential of a surface of a photosensitive member to a direct-current voltage value of the charging voltage applied to the charging member, and thus improving uniformity of the charging potential in the surface of the photosensitive member.
However, with variations in a resistance value caused by temperature changes in the charging member, such as the charging roller, the amount of a discharge current between the charging member and the photosensitive member varies. For example, in the case where the temperature of the charging member declines, the amount of the discharge current between the charging member and the photosensitive member may become insufficient and the charging potential of the surface of the photosensitive member may suffer from charging unevenness. For example, in the case where the temperature of the charging member rises, the amount of the discharge current between the charging member and the photosensitive member may become excessive, the generation of discharge products may increase, and image deletion may easily occur.
As described in Patent Literature 1, disposing a temperature sensor inside an image forming apparatus and controlling a charging voltage applied to a charging member based on a result of detection by the temperature sensor is known.
However, even in the case where the charging voltage is controlled based on the result of detection by the temperature sensor disposed inside the image forming apparatus, as described in Patent Literature 1, for example, if the charging member as at least part of a process cartridge as a unit is replaceable, a temperature detected by the in-device temperature sensor may differ from a real temperature of the charging member immediately after replacement of the charging member.
For example, in the case where a process cartridge retained in low-temperature environments in winter is newly attached to the image forming apparatus, the temperature of the charging member may be lower than the temperature inside the image forming apparatus, the amount of the discharge current between the charging member and the photosensitive member may become insufficient, and the charging potential of the surface of the photosensitive member may suffer from charging unevenness.